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Proportioning Concrete

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Proportioning Concrete

 

The process of selection of relative proportions of cement, sand, coarse aggregate and water, so as to obtain a concrete of desired quality is known as the proportioning concrete.

It is observed that if a vessel, is taken and filled with stones, of equal sizes, the voids to the extent of about 45 per cent are formed. This result is independent of the size of stones. It is interesting to note that if sand is taken in place of stones, the same result will be obtained.

The result can be verified by pouring water in the vessel till it is full. The volume of water added in the vessel represents the amount of voids.

The theory of formation of concrete is based on this phenomenon of formation of voids. When coarse aggregate is placed, such voids are formed. When fine aggregate i.e. sand is added, it occupies these voids. Further, when finely powdered cement is added, it occupies the voids of sand particles. Finally, when water is added, it occupies very fine voids between the cement particles. During the process of setting, a chemical reaction takes place between water and cement. This results n an absolutely solid substance, known as the concrete.

In general, the proportions of coarse aggregate, fine aggregate, cement and water should be such that the resulting concrete has the following properties:

(1) When concrete is fresh, it should have enough workability so that it can be placed in the formwork economically.

(2) The concrete must possess maximum density or in other words, it should be the strongest and most watertight.


(3) The cost of materials and labour required to form the concrete should be minimum.

There are various methods for determining the volumetric proportions of various components of concrete and their detailed discussion can be studies from any standard text-book on the design of R.C.C. structures. Following are the different methods of proportions concrete:

(1) Arbitrary method

In this method, there is no rigid control over the strength of the concrete mix. However this method is widely used used for all works of small magnitude because of its simplicity in the design.

The recommended mixes of concrete for various types of construction. The maximum size of aggregate is also mentioned in the table. The proportions are by volume.

The concrete as per BIS:456: 1978 is designated in seven grades namely, M10, M15, M20, M25, M30, M35 AND M40. The letter M refers to the mix and the number indicated the specified compressive strength of that mix at 28 days expressed in N/mm2.

For lean concrete bases and simple foundations for masonry walls, M5 and M7.5 grades of concrete may be used. These mixes need not be designed. The grades of concrete lower than M15 are not to be used in R.C.C. work. For general guidance, the nominal mixes correspond approximately to the different grades as follows:

M5 – 1: 5: 10 M10 – 1: 3: 6 M20 – 1: 1½ :3

M7.5 -- 1: 4: 8 M15 – 1:2:4 M25 – 1: 1: 2


(2) Fineness modulus method: In the fineness modulus method, the fineness modulus of sand and aggregates is determined by the standard tests. The term fineness modulus is used to indicate an index number which is roughly proportional to the average size of particle in the entire of aggregates.

 
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